You can query data in a database (ask it questions).
  You can look up data from a database relatively rapidly.
  You can relate data from two different tables together using JOINs.
  You can create meaningful reports from data in a database.
  Your data has a built-in structure to it.
  Information of a given type is always stored only once.
  Databases are ACID.
  Databases are fault-tolerant.
  Databases can handle very large data sets.
  Databases are concurrent; multiple users can use them at the same time without corrupting the data.
  Databases scale well.

  via http://programmers.stackexchange.com/questions/190482/why-use-a-database-instead-of-just-saving-your-data-to-disk